1. Field of the Invention
The present invention relates to a device and method to reduce the amount of ink used in a color inkjet printer. More particularly, the present invention relates to a device and method to reduce the amount of ink used in a color inkjet printer while maintaining a desired color lightness, chroma and hue value within a predetermined percentage deviation.
2. Description of the Related Art
In recent years color inkjet printers have been developed for home and office use. These printers have typically used three color inks comprising cyan, magenta, and yellow (hereinafter "CMY") color inks. In addition, a black (hereinafter "K") ink color is often used to form a four-color inkjet printer comprising cyan, magenta, yellow, and black (hereinafter "CMYK"). These color inkjet printers are suitable for generating reports, charts and graphs in color. However, several problems are frequently present in these printers caused by excessive ink usage. These problems include: ink smearing; ink bleed through; transfer of images to other sheets when stacked; high cost of printing color images; long drying times; and slow printing.
All of the above identified problems are related to the need to use large quantities of ink in order to create the more intense and darker color images. In the prior art, attempts have been made to resolve these problems by using faster drying inks and incorporating drying devices in the printers such as fans and heaters to more quickly dry the inks. However, these solutions have increased the cost of the printers by adding fans or heaters and have also increased the printing cost by using more expensive fast drying inks. Another solution employed has been to provide more time for the ink to dry by slowing the speed of printing. However, this is an unacceptable solution since users have grown accustomed to the speed of laser printers and are annoyed by the slow speed of printing using such a color inkjet printer. Further, this solution does not resolve the problem of bleed through seen in the prior art since the volume of ink used remains unchanged.
One of the reasons for these serious problems in the CMY or CMYK printing is that the substrate (especially plain paper) cannot absorb and sustain the volume of ink needed to create the more intense and dark color areas of an image. For example, the print medium in CMY printing may at most be able to accept 70% C, 70% M, and 70% Y ink being applied to the paper as the maximum amount of ink that can be applied. However, 70% maximum ink application of individual inks may not be enough for saturated cyan, magenta, or yellow color alone since the gamut or range of colors may be so reduced that a proper color image cannot be generated. In addition, it is not possible to simply reduce the amount of ink used uniformly throughout the entire color range of a color inkjet printer and achieve the same color lightness, chroma and hue desired.
Therefore, a need exits to reduce the volume of ink used in printing images and thereby eliminate the foregoing problems while at the same time maintaining the same color lightness, chroma and hue for the entire color range of the printer within an acceptable predetermined tolerance.
The concept of color lightness, chroma and hue was established and defined in the CIELAB color system, which was established by the Commission Internationale de I'Eclairage (CIE) in 1976. The CIELAB system is a color space used in specifying color differences. It consists of three variables comprising L* (color lightness), a* (red-to-green), and b* (yellow-to-blue) forming Cartesian coordinates in a three-dimensional color space as shown in FIG. 1. The L* (hereinafter "color lightness") value represents a perceived lightness ranging from 0.0 for black to 100.0 for a diffuse white. The a* value dimension represents the red-green perceived color. The b* value dimension represents the yellow-blue perceived color. The a* and b* values range from negative to positive values. Their maximum values are limited by the physical properties of the materials employed including the inks and print medium. The color difference (.DELTA.E*.sub.ab) between two color points, (L*.sub.1, a*.sub.1, b*.sub.1) and (L*.sub.2, a*.sub.2, b*.sub.2), is defined by the distance between the two points and is computed by EQU .DELTA.E*.sub.ab =[(L*.sub.2 -L*.sub.1).sup.2 +(a*.sub.2 -a*.sub.1).sup.2 +(b*.sub.2 -b*.sub.1).sup.2 ].sup.1/2
This color space can also be represented by cylindrical coordinates as shown in FIG. 2. The cylindrical coordinate system provides predictors of chroma, C*.sub.ab, and hue, h.sub.ab as expressed by EQU C*.sub.ab =[a*.sup.2 +b*.sup.2 ].sup.1/2
and EQU h.sub.ab =tan.sup.-1 (b*/a*).
In this instance, the chroma correlates the colorfulness of an area and the hue correlates the types of colors such as red, green, blue, yellow, etc.
Prior to the present invention, a technician would visually inspect dark color wedges produced by a printer through the range of intensities and reduce the ink levels until an acceptable level of color and bleed through are achieved. Unfortunately, this would not create consistent results from one technician to the next and it also took a great deal of time.
Therefore, a device and method is needed that will eliminate the problems created by the use of excessive amounts of ink while maintaining the same consistent color lightness, chroma and hue values or at least maintaining these values within an acceptable predetermined tolerance value.